1. Field of the Invention
The present invention relates to a semiconductive ceramic composition, and more specifically to a semiconductive ceramic composition having negative resistance-temperature characteristics. The present invention also relates to a semiconductive ceramic element making use of the semiconductive ceramic composition (a Negative Temperature Coefficient or "NTC" thermistor). The element is used, for example, for the prevention of rush current and the realization of "soft-starting" of a motor.
2. Background Art
The word "thermistor" is derived from "thermally sensitive resistor" and refers to an element whose resistance varies depending on temperature. An NTC thermistor is a kind of semiconductive ceramic element and has characteristics by which resistance decreases with increase of temperature.
The B constant is defined by the following equation, wherein .rho.(T) is the resistivity at temperature T, .rho.(T.sub.0) is the resistivity at temperature T.sub.0, and ln is natural logarithm. EQU B constant=[ln .rho.(T.sub.0)-ln .rho.(T)]/(1/T.sub.0 -1/T)
The greater the value of B constant, the greater the variation of resistance of an NTC thermistor per unit temperature change.
An NTC thermistor is incorporated into, for example, a rectification circuit for a power source in an electronic apparatus. The rectification circuit for the power source has a smoothing capacitor having a large capacitance. The NTC thermistor suppresses the strong rush of current flowing into the capacitor upon switching on the power. Thereafter, the thermistor comes to have lowered resistance through self-heating, so that the circuit operates at a steady state. The NTC thermistor is useful for achieving soft-starting of the circuit and for protecting the rectifier and the capacitor having large capacitance.
As a material for NTC thermistors, there has been used spinel composite oxides containing a transition metal element.
One requirement for an ideal NTC thermistor to prevent rush current is sufficiently low resistance of the thermistor at high temperature (about 140-200.degree. C.). As resistance of the thermistor decreases, power is conserved during steady state operation of the circuit. In order to decrease the resistance of the thermistor at high temperature, the B constant at high temperature is increased. Conventional NTC thermistors have a B constant value of 3250 K at most.
Another requirement is that the resistance of the thermistor at low temperature (in the range between about -10 and +60.degree. C.) must not be very high. A considerable increase of the resistance is observed for conventional NTC thermistors, especially at a low temperature below 0.degree. C. Consequently, a voltage drop occurs at low temperature to sometimes hamper normal starting-up of the electronic apparatus. In order to prevent an increase of the resistance at low temperature, the B constant at low temperature is reduced.
The B constant of a lanthanum cobalt oxide is reported to be temperature-dependent (see, for example, V. G. Bhide and D. S. Rajoria, Phys. Rev. B6, [3], 1072, 1972, etc.).
The present inventors previously obtained an NTC thermistor satisfying the two above requirements, i.e., having a B constant of about 3000 K or less near room temperature and a B constant of about 4000 K or more at high temperature, by incorporating at least one element selected from the group consisting of Si, Zr, Hf, Ta, Sn, Sb, W, Mo, Te and Ce into a primary component formed of lanthanum cobalt oxide (Japanese Patent Application Laid-Open (kokai) No. 7-176406). Resistivity and B constant of the obtained NTC thermistors are in proportion to the amount of the additive(s). This is because the additive functions as a donor within the lanthanum cobalt oxide to compensate for impurities (acceptors such as Ni, Ca, etc.) contained in the oxide. Therefore, if additives are incorporated in excessive amounts, the resistivity and B constant decrease at room temperature.
The peak values of the resistivity and B constant of lanthanum cobalt oxide at room temperature (25.degree. C.) to 140.degree. C. are approximately 20 .OMEGA..multidot.cm and approximately 4700 K. Conventionally, there have been obtained no NTC thermistors having a resistivity equal to or higher than the above value.
Depending on use of the NTC thermistor, a resistance equal to or higher than the above value may be required. Although higher resistivity may be obtained by increasing the volume of the NTC thermistor, an increase of the volume is contradictory to demands for smaller elements. The modification of the volume thereof in accordance with the type of thermistor invites an increase of manufacturing costs.
In the meantime, lanthanum cobalt oxide has extremely poor sinterability, and the sintered density thereof sometimes fails to reach 90% or more of the theoretical density. Heretofore customary sintering aids such as SiO.sub.2 have not been usable, since the balance between the donors and the acceptors--which exist in very small amounts--affects the resistance-temperature characteristics.